1. Field of the Invention
The invention relates generally to silicon micromachining, and in particular to micromachining torsion hinges that couple for relative rotation two members such as those included in optical beam vibratory scanners.
2. Description of the Prior Art
Beam scanners are used in digital imaging, printing, bar code readers, optical reading and writing systems, surface inspection devices and various scientific and industrial implements. Such scanners deflect a beam of light, usually from a fixed source, over an angle ranging from several degrees to tens of degrees. The beam sweeps back and forth at a frequency determined in part by the mirror resonant frequency. A typical vibrational scanner of the prior art is described in U.S. Pat. No. 4,732,440 to J. Gadhok. The idea of making torsional scanners within a silicon body was proposed at an early date by K. Peterson, Proc. IEEE, vol. 70, no. 5, p. 61, May 1982. See also U.S. Pat. No. 4,317,611 to K. Peterson.
FIG. 1, depicting a scanner shown in FIG. 39 of Peterson, Proc. IEEE, supra, p. 61, includes a micromachined torsional mirror 11, supported by torsion bars 13 and 15 within silicon body 17 (xe2x80x9cmicro scannerxe2x80x9d hereafter). The aforementioned article describes typical mirror parameters, such as the modulus of silicon, the typical wafer thickness, the length of the torsion bar and the dimensions of the mirror. The width of the torsion bars is on the order of 500 micrometers, while the length of the torsion bars is approximately 0.2 centimeters. The mirror is approximately 0.22 centimeters on a side. The cut which isolates the mirror from the silicon body and also defines the torsion bars is approximately 0.02 centimeters in thickness. Each cut is made by anisotropically etching the silicon. The silicon body rests on glass substrate 21 which has vapor deposited electrodes 23 and 25. A depression 27 is etched into the glass to receive silicon body 17 which rests on a linear support ridge 29. A high voltage is applied first to one electrode then the other in a continuing out-of-phase sequence from a drive circuit. The electric field generated by the electrodes tilts the mirror first to one side and then the other. The restoring force of the torsion bars works against each deflection. The resonant frequency of the mirror can be calculated with well known formulas cited in the above-mentioned articles, although air damping creates an error in the resonance frequency. The substrate, electrodes and drive circuit are part of the micro scanner.
Two dimensional micromachined silicon flexure structures, used as gyroscopes, are known in the art. See U.S. Pat. No. 5,016,072 to P. Greiff. Such structures are similar to micro scanners in construction and vibratory characteristics.
One of the problems encountered in the prior art is in restricting vibrations to a single desired torsional mode. An object of the invention was to devise a micro scanner which vibrates at a single desired mode of vibration and to be self-oscillating at its natural fundamental frequency. Another difficulty with the prior art structures and fabrication methods is an inability to control, balance, or eliminate stress in micromachined plates or frames. Yet another difficulty encountered in fabricating micro scanners is obtaining very high quality mirrors and torsion bars that have a specified thickness and, and, if desired, that are extremely thin.
An object of the present invention is to provide optically flat micromachined plates and frames coupled to each other by torsional hinges.
Another object of the present invention is to provide micromachined plates and frames coupled to each other by stress free torsion hinges.
Briefly, the present invention is a method for fabricating an integrated, micromachined structure, such as a torsional scanner, that includes, a reference member, such as a frame, a pair of torsion hinges that project from the reference member. The micromachined structure also includes a dynamic member that is coupled to the reference member by the torsion hinges to be thereby supported from the reference member for rotation about an axis.
The method for forming the micromachined structure includes providing a wafer that has been formed from silicon material, and that has both a frontside and a backside. A membrane is formed in the wafer by etching a cavity in the silicon material from the backside of the wafer. The method also includes establishing a pattern that defines the mirror surface and the torsion hinges on the frontside of the wafer at the membrane formed therein. The frontside of the wafer is processed to form therein the dynamic member and the torsion hinges that support the dynamic member for rotation about the axis.